The invention relates to a method and a device for catalytic reduction of nitrogen oxides for mobile applications and, more particularly, to a method for reducing nitrogen oxides by reduction on a catalyst, with the hydrogen required for nitrogen reduction being generated aboard the motor vehicle by electrolysis.
For the operation of motor vehicles with gasoline and, especially, diesel engines, the observation of the applicable legal emission guidelines is indispensable. In this connection, catalytic NO.sub.x reduction using hydrogen is being used advantageously.
This catalytic removal of nitrogen oxides from the combustion exhaust from motor vehicles is performed using hydrogen on suitable catalysts with the reaction EQU 2NO+2H.sub.2 .fwdarw.N.sub.2 +2H.sub.2 O.
In the known method for removing nitrogen oxides by NO.sub.x reduction, the hydrogen required for the reaction is carried in the vehicle, for example, in compressed gas tanks, liquid hydrogen tanks, or metal hydride storage devices. The disadvantage of this method is that large heavy containers are required to transport the hydrogen. These cumbersome containers also have a narrowly limited capacity, hence requiring short intervals between refills. In addition, the filling or replacement of H.sub.2 tanks as well as the presence of such H.sub.2 tanks poses a safety problem in and of itself.
A method for NO reduction of exhaust in a motor vehicle is known from European Patent document EP 0 566 071 A1, in which the required hydrogen is generated aboard the motor vehicle by electrolysis. Electrolysis is performed on an arrangement of electrodes between which a proton-conducting solid electrolyte is located. The anode consists of an oxidation catalyst and the cathode consists of a reduction catalyst. NO reduction takes place directly at the cathode with the participation of the hydrogen which penetrates the solid electrolyte.
One disadvantage of this known method is that the catalyst must be heated to a reaction temperature of at least 300.degree. C. and is therefore not practical for gasoline and diesel engines, especially during the starting processes and in the partial load range. The exhaust temperatures in the partial load range decrease to about 100.degree. C. at the point where the exhaust aftertreatment system is located within the motor vehicle. Therefore, systems that operate above approximately 100.degree. C. require additional heating.
Another disadvantage of the known method is that the method has a low efficiency. The H.sub.2 gas stream that is generated is less than 1 ml/min. In the vehicle, however, a H.sub.2 gas stream of at least 0.2-0.7 l/min is required. If it were desired to use this method in a conventional diesel-powered vehicle, an electrode with a diameter of more than 1 meter (approximately 3 m.sup.2 electrode surface) would be required.
German Patent document DE 42 30 408 A1 describes a method for NO reduction of exhaust in an internal combustion engine in which the required hydrogen is generated by electrolysis of water using a proton-conducting membrane.
This method has the following disadvantages: 1) under freezing conditions, additional heating of the electrolyzer is required to keep the membrane from being destroyed; 2) high requirements are imposed on the water quality (deionized water is required); and 3) the methodology is very expensive since the water can only be introduced through a circuit charged with water vapor.
There is therefore needed a method for NO.sub.x reduction using hydrogen in a motor vehicle in which compact, energy-efficient, mechanically loadable, and inexpensive hydrogen generation can be performed directly aboard the motor vehicle.
These needs are met according to the present invention by a method for reducing nitrogen oxides by reduction on a catalyst, with the hydrogen required for nitrogen reduction being generated aboard the motor vehicle by electrolysis. The electrolysis is performed using a liquid fixed electrolyte. These needs are further met by a device for reduction of nitrogen oxides in motor vehicle exhaust by catalytic reduction, including a reactor in which a catalyst is contained, on which nitrogen oxide reduction is performed with the addition of hydrogen, and at least one electrolyzer for generating hydrogen aboard the motor vehicle. The electrolyzer includes an electrolyte with a liquid fixed electrolyte.
According to the present invention, the hydrogen required for NO reduction is generated aboard the motor vehicle by electrolysis using a liquid fixed electrolyte. An alkaline electrolyte is preferably used for this purpose. This type of electrolysis is referred to in the following as LAE electrolysis (L=liquid, A=alkaline, E=electrolyte).
The generation of hydrogen aboard a motor vehicle with a fixed, especially an alkaline electrolyte, has the following advantages: 1) good suitability for cyclic operation; 2) only minor requirements are imposed on water quality; 3) the H.sub.2 generator is suitable for operation in moving systems without additional devices such as, for example, separators; 4) low electrical power requirement due to the high efficiency; 5) the manufacture of the H.sub.2 generator by mass production is inexpensive because of the simple manufacturing steps and the commercially available materials; 6) simple methodology; and 7) adaptation to the required geometry is readily possible.
The electrolysis of water is performed by the relationship EQU H.sub.2 O+electrical energy.fwdarw.H.sub.2 +1/2O.sub.2 +heat
in a plurality of electrolysis cells connected in parallel. The operation of the electrolyzer uses thermoneutral voltage, so that heat is produced as well. The electrical power required for generating hydrogen can be obtained from the alternator in the motor vehicle. In all, a maximum electrical power requirement of 190 W is needed for complete reduction of nitrogen oxides in today's automobile diesel engine exhaust. This peak power requirement however is sharply reduced by an electrolyzer gas-pressure reduction. The average electrical power requirement can be considerably lowered by the short response times of electrolysis by demand-oriented H.sub.2 generation using an already existing accelerator potentiometer.
The relatively low water requirement is met by a separate water container. Advantageously, the water can also be supplied through the water-vapor-charged engine exhaust from the motor vehicle.
It is a further advantage of the present invention that separate tempering for the electrolyzer is not required. Heat exchange with the environment is sufficient because of the simple and flexible operating mode of the electrolyzer. The operating temperature range of the electrolyzer is between -25.degree. C. and +95.degree. C.
The electrolyzer can be operated at variable pressure, so that very rapid load changes are advantageously possible.
In one especially advantageous embodiment, the hydrogen, in addition to electrolysis using fixed liquid solid electrolyte, is also generated by water vapor reformation and/or partial oxidation of hydrocarbons, for example methanol, diesel fuel, or gasoline. Hydrogen generation by partial oxidation or reformation of hydrocarbons aboard a motor vehicle in and of itself is known from European Patent document EP 0 537 968 A1.
In this case, the device for generating hydrogen comprises, in addition to an electrolyzer with a liquid fixed solid electrolyte, a reactor for water vapor reformation and/or a reactor for partial oxidation of hydrocarbons. Such hybrid systems are especially advantageous for applications that have a high hydrogen requirement and require short response times for a cold start and load changes.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.